FSK AND PSK IN OPTICAL COMMUNICATION NETWORK

Prajwal Doddaballapura LakshmijayasimhaMatriculation Number: 33267852

Date: 10/06/2014

Supervised by: Prof. Axel Bangert 10/6

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OUTLINE

INTRODUCTION MODULATION FORMATS COMPARSION MODULATORS PERFORMANCE CONCLUSION REFERENCES

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INTRODUCTION

• OPTICAL NETWORK ? Optical network can be defined as a data network built on fiber optics technology, which sends data digitally, as light through connected fiber strands.

• MODULATION ? Conversion of digital data in electronic form to an optical signal.

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[2]

MODULATION FORMATS

• Amplitude-Shift-Keying (ASK) is the technique of modulating the intensity of the carrier signal.

• On and off states are represented by the presence or absence of light respectively.

• In ASK both frequency and phase remain constant, while amplitude changes.

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Baseband Data

Acos(t) Acos(t)

1 0 0 1

1. ASK

Fig1:ASK signal output

• Frequency of a laser light is switched between two frequencies.

• The shift  Δf = Δω/2π is called

frequency deviation

• The frequency of the light-wave changes by  Δf.

f+Δf for logic “1” ; f-Δf for logic “0”

• Total bandwidth of the FSK signal is approximated to 2Δf+2B.

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Es(T) = AsCos [(Wo ±ΔW)t + φs ]

• When Δf>>B ,then bandwidth approaches 2Δf : wideband FSK.

• When Δf<<B ,then bandwidth approaches 2B : narrow band FSK.

The optical field for FSK format can be written as

2. Frequency Shift Keying

[2] Fig2 :FSK signal output

3. Phase Shift Keying

• Modulates the phase of a light beam (the carrier) while the frequency & amplitude remains constant.

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• BPSK: two phases represent two binary digits.• DPSK: Phase shifted relative to previous transmission

rather than some reference signal.

• For binary PSK, the phase is 0° or 180°.• For multilevel PSK, the change may be in increments for example, 45° (8 levels).

Seminar on Optical Communication Systems 6[1] Fig3: BPSK signal output

COMPARSION

FSK vs PSK • less susceptible to interference .• easier to generate with modern electronics• BFSK requires twice the bandwidth of BPSK• Power FSK is better compared to PSK but it is expensive ASK• Simplicity• low implementation costs• conserving power.

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FSK MODULATOR

Fig 4: FSK modulator

• The FSK modulator consists of 4 optical phase modulators.

(pair of Mach-Zehnder structures )

• When we apply a pair of RF signal which are of same frequency fm and have 90° phase difference, to the electrode RFA and RFB, frequency shifted light wave can be generated.

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[5]

FSK MODULATOR

• When the phase difference induced by RFC is -90° or +90°, we can get carrier suppressed single sideband modulation ( LSB or USB).

• When the FSK switching signal fed to RFC is in “1” state

the output carrier frequency is f0 + fm (USB).

• When the FSK switching signal fed to RFC is in “0” state

the output carrier frequency is f0 - fm (LSB).

Where f0 the frequencies of input light wave.

fm the frequencies of RF Signal.

• Thus, the optical frequency of output light wave can be switched by changing the induced phase at RFC. Seminar on Optical Communication Systems 9

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FSK MODULATOR

For instance , Due to 90° phase differences in the RF-signal and the light wave, the polarity of LSB component at point “P” is the opposite of “Q.” The LSB would be vanished at the output port.

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[5] Fig 5: Demonstrates USB generation

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[5] Fig6 : Principle of FSK signal Transmission

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• The IM/FSK signal is fed to an intensity modulator which is in null-bias point to obtain DSB-SC modulation.

• The output has three optical spectral components whose carriers are f0 -2fm, fm, f0 +2fm.

• When FSK switching signal is at • ‘1’ state the output of the intensity modulator has two

carriers f0 , f0 +2fm • ‘0’ state the output of the intensity modulator has two

carriers f0 , f0 -2fm • The output always has the f0 component regardless of the

FSK switching signal.• Thus by using optical band pass filter whose center frequency

is f0 , we can remove the FSK signal from the IM/FSK signal, and restore a pure IM signal which does not have an FSK signal.

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PSK MODULATOR

• PSK is implemented externally by passing the light

beam through a device known as an electro-refraction

modulator.

• Lithium Niobate (LiNbO3) is the most commonly used

electro-optical crystal to fabricate external modulator.

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Fig 7: PSK external modulator

• When voltage is applied to the crystal ,the electric field of E is equal to V/d.where is the distance between the two electrodes.𝑑

• The refractive index of the material is changed. δn / 𝛼 𝐸 𝛼 𝑉 𝑑

• A phase shift of π can be imposed on the optical carrier by applying the required voltage for the duration of each “0” bit

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PSK MODULATOR

[1]

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PSK MODULATOR

• The phase difference is expressed by

Where λ wavelength of the optical signal. Lm length over which the index changes by

the applied voltage.

[1]

PERFORMANCE PSK vs FSK

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Fig 8 : BER vs EIRP for a practical PSK system Fig 9: BER vs EIRP for a practical FSK system[7]

PSK vs FSK

• On comparing, FSK system presents a better BER than the PSK system for the same EIRP values.

When EIRP increases, the BER of the FSK system drops at a much higher rate than the BER of the PSK system. At an EIRP of 50 dB, the 16PSK system's BER is 10-

2 , while that of the 16FSK system is only 10-22 .

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CONCLUSION

The comparison shows that, as the value of Gain (M)

increases, the BER of PSK system increases, while the BER of

an FSK system decreases. We have concluded that BPSK,

QPSK or 8FSK are the most advantageous modulation

schemes in a practical communication system.

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REFERENCES

[1] Govind P. Agrawal, Fiber-Optic Communications Systems, 3rd ed., Wiley& Sons, 2002 [2] Optical Networks-A Practical Perspective(Third Edition)[3] Keang-Po Ho, Phase-Modulated Optical Communication Systems, Springer, 2005 [4] IOSR Journal of Electronics and Communication Engineering (IOSR-JECE) [5] Journal of Light wave Technology, Vol. 23, No. 1, Jan 2005 [6] IJCEM International Journal of Computational Engineering & Management, Vol. 13, July 2011 [7] Journal - PIERS Proceedings, Cambridge, USA, July 5{8,

2010

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THANK YOU

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